Polymethylmethacrylate (PMMA) resins have long been used for the manufacture of contact lenses because of their excellent optical properties and machining and molding characteristics. A major disadvantage of PMMA resins is their very low permeability to gases such as oxygen present in the air and carbon dioxide that is a metabolic waste product produced by the eye. Since the cornea needs a continuous supply of oxygen from the air to provide for ongoing metabolic processes, the low gas permeability of the PMMA resins has necessitated lens designs which ameliorate this problem to some degree. Design changes have included reducing the diameter of the lenses in order to decrease the amount of corneal area covered by the impermeable material and shaping the back surface of the PMMA contact lens to provide for a pumping action and concomitant tear flow under the lens, the tears containing dissolved oxygen from the air.
While such designs have made possible the wearing of contact lenses, significant problems and limitations remain, both because of the inadequacy of the oxygen supply to the cornea and because the designs may produce discomfort and undesirable physiological symptoms to the wearer, frequently to a degree which makes wearing of the contact lens possible for only short periods of time or not at all.
Continued oxygen deprivation of the cornea results in edema or swelling of the cornea which may result in corneal damage. In addition, while oxygen must be supplied to the cornea for its metabolic processes, carbon dioxide, a waste product of these processes must be removed. The same principles apply for providing a route for removal of carbon dioxide from the cornea as for the transport of oxygen to the cornea, when a contact lens covers the cornea. As used herein, the term "gas permeable" encompasses permeation of said gases through the lens.
The ideal material would provide oxygen transport to the cornea equivalent to that without a lens present on the cornea. It has been found, however, that the cornea can remain healthy with an oxygen delivery lower than this, provided the continuous lens wear time is appropriately curtailed. It has been well established, however, that the higher the gas permeability, the greater the safety margin for retaining a healthy cornea, the greater the patient tolerance for the lens and the longer the continuous wear time of the lens by the patient.
Polymethyl methacrylate, which is the polymer of which most non-hydrophilic or rigid lenses currently in use are made, has a permeability constant (P value as hereinafter defined) of about 0.1.times.10.sup.-11 and must have an efficient mechanical pump design to permit wear for even short periods of time. Recently, cellulose acetate butyrate with a P value of about 4.times.10.sup.-11 has been used as a rigid contact lens material. This lens permits much longer wear than the PMMA lens, but has the disadvantage that the lens material is unstable and changes its shape. Polysiloxane rubber contact lenses have very high P values, of the order of 70-80.times.10.sup.-11, but the surface of such lenses are very hydrophobic necessitating hydrophilic coating or surface treatments which have not been found to be permanent. Further, the polysiloxane lenses appear to enhance solid deposits thereon from the tear fluid thereby reducing optical clarity. Other recent contact lens materials such as copolymers made of methyl methacrylate and silicone methacrylate monomers (U.S. Pat. No. 3,808,178), or methyl methacrylate and fluoro methacrylate monomers (U.S. Pat. No. 3,950,315) also have enhanced P values, but suffer from lack of surface wettability and difficulty in making the polymers.